1. Field of the Invention
The present invention relates to dynamic MOS memories, and more particularly, to a method of refreshing the contents of such memories.
2. The Prior Art
Dynamic MOS memories must be refreshed periodically in order to maintain the information in storage. This is because the operative storage elements function as capacitors which are charged in accordance with the information stored in the memory locations. Due to leakage currents, the capacitors gradually lose their charges if they are not periodically refreshed. Refreshment of the stored information is accomplished by a normal read process. and such process must be performed for each individual storage location periodically, at least once during an interval hereinafter called the refreshment period T. During refreshment, normal access to the memory for reading and writing is blocked. Any attempt to read or write during the refreshment period is delayed, so that the mean access time of the memory is extended.
When the refreshment period T is 2 milliseconds and the cycle time of the memory (for reading and writing) is 0.6 microseconds, the mean access time for a memory having 512 refresh locations is extended by approximately 15 percent This is because approximately 15 percent of the time is devoted to refreshment, during which normal access is blocked. For a memory having four times the capacity, the mean access time is extended by about 60 percent. This constitutes a serious disadvantage because it is desirable to maintain the means access time of a memory at the lowest possible value, in order to maximize the speed of operation.